In the continuous casting of ferrous and non-ferrous metals there has been developed caterpillar type molds in which the casting mold is formed by a double row of mold halves which are connected into a pair of endless circularly running chains. At the inlet end the mold halves, which are positioned opposite each other, lie against each other and move in such position over a certain distance over which they form the caterpillar type mold itself. The molds then separate and after a short time come together again at the inlet end.
Of the machines designed for casting aluminium and aluminium alloy plate or strip the one designed by the Hunter Douglas Corporation has been employed for some 20 years. In the Hunter Douglas caster the plate or strip is cast in the horizontal plane. The liquid is fed to the caster via a flat-shaped nozzle made of fire-resistant material. The fire-resistant material is made of a mixture of 30% long asbestos fibers, 20% sodium silicate (dry weight) and 28% chalk to form calcium silicate which is more heat resistant than the sodium silicate. The sodium silicate is added to the other components as water glass, to form a doughy mass which is shaped and then baked under slight pressure. The process for manufacturing this material is described in the U.S. Pat. No. 2,326,516.
More recently another caterpillar type caster for casting wide metal strip has become available. The machine casts in the horizontal plane or in a plane slightly inclined to the horizontal. A nozzle for feeding liquid metal to this machine is described in the Swiss Pat. CH-PS No. 508,433.
In order to transfer the molten metal to the mold when casting strip in caterpillar type molds which are horizontal or slightly inclined to the horizontal, known metal transfer nozzles which practically seal off the cross section of the mold must be under a certain metallostatic pressure from the trough. This is the case with the transfer nozzle in U.S. Pat. No. 2,752,649 and those nozzles in Swiss Pat. CH-PS No. 508,433 and U.S. Pat. No. 3,774,670.
Known methods employ flat nozzles in slightly inclined caterpillar type molds. The mold operates as a furnace dependent mold where the term furnace is to include all kinds of containers for the melt i.e. the trough. During casting the caterpillar type mold is always kept filled to where the nozzle tapers so that during casting no metal head with "free" surface forms on the nozzle.
The process of the present invention for the delivery of molten metal during the casting of wide strip non-ferromagnetic metals such as aluminum and aluminum alloys, copper and copper alloys, zinc and zinc alloys and magnesium and magnesium alloys, into a caterpillar type mold inclined at 3.degree. to 30.degree. to the horizontal via a nozzle made of fire resistant material is such that the molten metal is led into a head of liquid metal in the mold and under the surface of the metal and in such a way as to avoid premature contact with the wall of the moving mold halves and so as to reach the wall of the mold through this metal head without almost any overpressure.
In contrast to the mode of operation used heretofore, in the process of the present invention a metal head with free metal surface as in DC casting is produced at the pouring end of the delivery nozzle. The caterpillar type mold inclined to the horizontal is used therefore, in terms of the present invention, as a furnace dependent mould. The metal head with free metal surface forms in the mold which is closed off at the start of casting by a dummy or starting ingot which is drawn out of the mold during casting after the head of metal has formed.
In known furnace dependent caterpillar type molds which are horizontal or lie at an acute angle of for example 1.degree. to 30.degree. to the horizontal, there is a free metal surface only in the trough but not in the mould itself.
Extensive trials by the inventor have shown that the metallostatic pressure in the caterpillar type caster inclined slightly to the horizontal can be detrimental on casting pure grades of aluminium using the practice employed to date which permits a free metal surface only in the trough thereby resulting in a, fast rate of cooling, one would not normally expect due to the temperature condition. It is preferred that the initial rate of solidification not be too fast.
Metal transfer without metallostatic pressure, as in the process of the present invention, causes the solidifying melt to exert less pressure on the surface of the mold which results in a lower rate of solidification at the start of casting. When rapidly cooled a metal crust form which exhibits stresses which cause buckling and consequently localised lifting away from the surface of the mold. At the places where the crust has lifted away cavities and pronounced porosity form in the cast strip.
The rate of solidification can be determined metallographically by measuring the fineness of the cellular structure.
In caterpillar type casters with mold halves made of steel, during the casting of a hot aluminium melt of purity 99.2% at 680.degree. to 700.degree. C. the mold walls have a temperature of, for example, 105.degree. to 115.degree. C., and on casting with closed metal delivery nozzles, as is described in the Swiss Pat. CH-PS No. 508,433, a structure of cell size 5 to 30 .mu.m is obtained up to a depth of approx. 0.3 mm below the outer skin. By using a closed nozzle the metallostatic pressure of the melt in the trough produces rapid cooling inside the mould thus causing the above mentioned disadvantages. If however the aluminum melt is allowed to flow from the trough to the mold without metallostatic pressure a structure of cell size 30 to 70 .mu.m is obtained to a depth of about 0.3 mm below the skin as a result of the slower solidification rate thus avoiding the above mentioned disadvantages.
Heretofore, it was not known nor expected that the metallostatic pressure in a caterpillar type caster, inclined at an angle of for example 1.degree. to 15.degree. to the horizontal, had such a pronounced effect on the rate of initial solidification.
Similar results are obtained with aluminium of other grades of purity with aluminium alloys, and with other non-ferromagnetic metals such as magnesium, zinc, copper and their alloys.
Attempts were made to decrease the rate of cooling in caterpillar type casters by appliying a coating to the walls of the mold halves of materials of lower heat conductivity than steel such as chrome-nickel steel and grey cast iron. Also, to decrease the rate of cooling higher mold temperatures were employed. While these measures did lead to less rapid solidification they also led to surface flaws in the cast strip. These surface flaws were mainly due to bleeding.
For reasons of simplicity metal delivery without a metallostatic head may be characterized as "almost pressureless delivery". In the trials carried out it was found that such almost pressureless delivery not only markedly reduced the solidification stresses in the strip, which can lead to cracks, but also resulted in the surface of the strip having much less or even no surface bleeding. Such surface bleeding causes steakiness in the sheet after it has been hot rolled.